1. Field of the Invention
The present invention relates generally to the fabrication of semiconductors, and more particularly, to the determination of in situ particle contamination levels and identities during ion implantation.
2. State of the Art
Ion implantation devices, or ion implanters, are known machines used in the fabrication of semiconductor chips. These devices are typically used to change the characteristics of a silicon wafer by inserting, or implanting, a thin layer of atoms into the wafer. This process is often referred to as doping, with the implanted atoms being referred to as the dopant.
Typically, doping is achieved by accelerating the dopant atoms to approximate speeds of one million miles per hour and bombarding the wafer surface with the accelerated ions. By regulating the speed of the dopant atoms, the number of implanted atoms and their depth of penetration into the silicon wafer can be accurately regulated. Common dopants include boron, phosphorous and arsenic.
One known ion implanter is the Varian Ion lmplanter. This device performs four basic steps to effect ion implantation. First, the ion implanter converts gas molecules into an ion beam. The device then selectively redirects specified ions in the beam so that they may be used for implant. Afterward, the specified ions are accelerated to a speed commensurate with desired implantation depth. Finally, the ion implanter controls the total number of ions implanted (i.e., the "dose").
A generalized block diagram of a typical ion implanter is shown in FIG. 1 to include three basic components: a source 2, a beam line 4, and an end station 6, all of which are vacuum pumped. The source 2 receives gas via a dopant feed 8. Ions are extracted out of the source using a potential difference of, for example, 25 keV to attract positively charged ions toward a mass analyzer 10. The mass analyzer uses magnetic forces to select ions having a desired mass and charge from undesired ions.
More particularly, the mass analyzer typically causes all ions in the beam to be deflected by an amount dependent upon their mass and charge. Desired ions having a specified mass and charge are deflected by an amount which enables them to pass through an aperture 12 located on a downstream side of the mass analyzer. An ion beam containing desired implantation ions is thus produced at a output of the aperture.
As mentioned previously, implanting of the selected ions is controlled by accelerating the ions to a desired speed. In the FIG. 1 implanter, ion speed is controlled by subjecting the ion beam to a high voltage coil 14. The high voltage coil 14 typically generates approximately 175 keV of energy to accelerate the ions. This value is adjusted up to, for example, 200 keV to increase ion speed and implant energy, and is adjusted downward to decrease ion speed.
The beam of accelerating ions leaving the source 8 is then focused in the beam line 4 via use of quadrapole lenses 16 and 18. Each of the quadrapole lenses surrounds the ion beam with, for example, two opposing positive lenses and two opposing negative lenses such that the ion beam passes between all four lenses. In the quadrapole lens 16, the opposing negative lenses are oriented vertically to permit focusing of the positively charged ion beam in a vertical direction. In the quadrapole lens 18, the opposing negative lenses are oriented horizontally to permit focusing of the ion beam in a horizontal direction. By changing the relative positive and negative potential of a given quadrapole lens, the orientation of the ion beam can thus be regulated.
The focused ion beam is then moved up and down and back and forth in a scanning motion via a charged scanner 20. Ions output from the scanner 20 are directed toward a silicon wafer located on a wafer plate 32 in the end station 6. Once implanted in a selected area of a wafer, the ions are converted into atoms which form a thin layer in the wafer.
While implanters of the above-described type are well known and widely used, they often result in the production of wafers containing contaminates Such contamination is, for example, the result of macro-contaminants present on the wafer prior to processing.
Macro-contaminants are also produced when gas deposits which have built up on walls of the wafer processing chamber flake off into the vicinity of the wafer. Further, differentially pumped vacuum seals used to seal off the stages of the implanter during processing slide against portions of the chamber, with attendant friction resulting in the release of particulate contaminates. During venting as well as during subsequent repumping of the chamber to processing pressure, turbulence and/or high energy states are created which can further accelerate and disperse particles.
Similar contaminates are introduced as a result of any friction created in the chamber. For example, implanters which use a rotary drive to turn a plate supporting several (e.g., 10) wafers in the end station will produce friction and release particle contaminants from ferromagnetic fluids used to seal the rotary drive. Further, if a wafer being processed is broken, contaminants are released.
In the past, efforts have been made to detect contaminants which may result in the production of defective wafers. For example, laser particle detectors have been provided for this purpose. Laser particle detectors sense contaminants by scanning a laser beam in an area of the end station where the wafer is located. Light reflected from particles which intercept the scanning laser beam are detected and used to increment a particle counter.
Known contaminant detectors such as the laser particle detector suffer from several disadvantages. For example, a key disadvantage of these detectors is that a particle must intersect the laser beam to be detected. Further, these detectors can not be placed near the wafer location in an implanter, but rather are located in a roughing line of the end station, approximately 15 inches away from the wafer. This separation distance creates additional inaccuracies in the measurement, as contaminants close to the wafer surface are often missed.
In addition, laser particle detectors are operated only during the pump and vent cycles of the end station, thus further reducing the detection accuracy. These detectors do not measure particles during actual implantation primarily because they rely on the pump and vent cycle to create turbulence in the end station.
Accordingly, there is a need to provide an accurate system and method for detecting the existence of contaminants present in an end station during wafer processing.